1. Field of the Invention
The present invention relates to an inorganic scintillator, and to a radiation detector and PET apparatus employing it.
2. Related Background Art
In Positron Emission computed Tomography (hereinafter, PET) apparatuses, the optical characteristics (wavelength conversion properties, etc.) of the scintillators mounted therein have a major influence on the overall apparatus imaging performance. Thus, improvement in the optical characteristics of the scintillator component is one of the most import aspects for enhancing overall apparatus imaging performance. Researchers are therefore actively exploring scintillator materials which can be used to construct scintillators with excellent optical characteristics, and are developing manufacturing techniques such as crystal growth techniques for realizing such scintillators.
PET examination can provide highly detailed information for cancer detection, and its clinical application for cancer is becoming more common. Clinical applications for cancer include, for example, diagnosis of tumor malignancy before treatment, clinical stage diagnosis based on detection of cancer infiltration extent and identification of metastatic lesions, judgment and evaluation of response to cancer therapy either during or immediately after treatment, and prognosis or diagnosis of recurrence after treatment. However, when the goal is to perform accurate diagnosis of the extent of tumor infiltration, PET images alone do not provide accurate positional data for biological organs or tissue.
On the other hand, X-ray CT or MRI apparatuses allow accurate visualization of detailed anatomical information from the body, and are widely used in the field of medicine. However, such apparatuses are not capable of analysis of metabolic function as is possible with PET. New cancer diagnosis methods have therefore been desired which take advantage of the excellent features of both PET apparatuses and X-ray CT or MRI apparatuses while compensating for their drawbacks.
In recent years, PET-CT apparatuses have been developed that simultaneously collect metabolic function information by PET imaging and anatomical position information by X-ray CT imaging, to permit diagnosis based on combination of both images. PET-MRI apparatuses are also being investigated, for simultaneous collection of metabolic function information by PET imaging and anatomical position information by MRI imaging, to permit diagnosis based on combination of both images.
However, because MRI requires a powerful magnetic field, the photoelectric multipliers employed in conventional PET apparatuses cannot be used. Specifically, a photoelectric multiplier converts fluorescent light emitted upon radiation incident onto the scintillator into an electrical signal, but for reasons of construction the photoelectric multiplier cannot be used in the presence of a powerful magnetic field. Photodiodes, on the other hand, are elements capable of converting fluorescence emitted from a scintillator into an electrical signal without being affected by magnetic fields.
In a radiation detector combining a photoelectric multiplier and a scintillator, the fluorescent wavelength with the highest conversion efficiency by the photoelectric multiplier to electrical signals is about 415 nm. Thus, radiation detectors employ scintillators having a light intensity spectrum peak wavelength of about 415 nm (for example, Japanese Examined Patent Application Publication HEI No. 7-78215). However, the wavelength region of high conversion efficiency for a photodiode is 450-600 nm. Consequently, a scintillator used to construct a radiation detector in combination with a photodiode preferably is one having a light intensity peak wavelength in the wavelength range of high conversion efficiency by the photodiode.
Japanese Patent Publication Laid-open No. 2001-4753 discloses a scintillator having a light intensity peak wavelength in the range of 450-600 nm, which is the region of high photodiode conversion efficiency. The scintillator described in Japanese Patent Publication Laid-open No. 2001-4753 is an oxide fluorescent material having a garnet crystal structure, with oxides composed of at least the elements Gd, Ce, Al, Ga and O.